The Physical Origin of the Scattering Polarization of the Na I D-Lines in the Presence of Weak Magnetic Fields
Javier Trujillo-Bueno, Roberto Casini, Marco Landolfi, Egidio Landi Degl'Innocenti
Abstract
We demonstrate that the atomic alignment of the hyperfine-structure components of the ground level S_{1/2} of Na {\sc i} and of the upper level P_{1/2} of the D_1 line are practically negligible for magnetic strengths B>10 \rm G, and virtually zero for B\ga 100 \rm G. This occurs independently of the magnetic-field inclination on the stellar surface (also, in particular, for vertical fields). Consequently, the characteristic antisymmetric linear-polarization signature of the scattered light in the D_1 line is practically suppressed in the presence of magnetic fields larger than 10 G, regardless of their inclination. Remarkably, we find that the scattering polarization amplitude of the D_2 line increases steadily with the magnetic strength, for vertical fields above 10 G, while the contribution of alignment to the polarization of the D_1 line rapidly decreases. Therefore, we suggest that spectropolarimetric observations of the ``quiet'' solar chromosphere showing significant linear polarization peaks in both D_1 and D_2 cannot be interpreted in terms of one-component magnetic field models, implying that the magnetic structuring of the solar chromosphere could be substantially more complex than previously thought.